Stabilized oscillator



July 3, 1945. w. A. EpsoN 2,379,694 1 STABILIZED OSCILLATOR Original Filed Jan. 16, 1942 I 3 Sheets-Sheet 2 FIG. 7

l8 I60 n I80 INVENTOR WA. EDSON .J 3, 11945. w. A. EDSON 2,379,594

STABILIZED OSCILLATOR Original Filed Jan. 16, 1942 3 Sheets-Sheet 3 FIG. I?

a w m RHQQGMS w INVENTOR WA; EDSO/V 8V $1 T Patented July 3, 1945 (2,319,6 4. s'rABn zEn osomLA'roa William A. nds on; NewgYork, N. Bell Telephone, Laboratories,

Yr, assignor to Incorporated,

. New York,QN. 31., a corporation of New York Originalapplication January IG, 1942, Serial No.

i a 427,031, n'ow'Patent No. 2,343,539, dated March a 7, 1944. Dividedand this application w, 1943, SerialNol 505,314

sciai nsg,iotzmgatl i This invention relates tooscillators'which rare stable as to' frequency,frequencystability being 1 conceived broadly i as inconsistentcrwith the presence. of certain phenomena that .commonlyrattend the generation of electric waves by practi cable means and result in waves which areother than. continuous sinusoidal waves. "This application is .a division of applicationSerial "No. 427,031, filed January16, *1942; corresponding to Patent No. 2,343,539, issuedMarch 7, 1944.; m

These phenomena include low frequency. disturbances, sometimes expressed, as interruptions of the primewave and sometimes as a fairly regular low frequency modulation ,of such :prime wave. These. and other'eifects contemplated occur inall feedback oscillators, to which class of oscillators the invention pertains, but some tend to beinduced more particularlywor at leastto a greater extent, in specific formsnof feedback oscillators and particularly in oscillatorswherein the invention teaching there is a physical separation of;.the elements.

which contribute the gain and amplitudelimitation functions, although the basic principles of the invention are not solimited andthe. in-

vention as expressed in other species is adapted toa circuit which does not satisfy this condition. It is an object of j the invention toreorganize thecircuits of old types of .osci1lat0rs,-So asito.

eliminate the low frequency disturbances that ate tend their normal operation. By themethod of analysishere presented, it is possible to determine the conditions for such disturbances. and the type of reorganization necessary .forelimj inating them. The eliminating means, a and thereforethe solution ofzthe problem imposed,

affects an auxiliary feedback circuit for gain or volume control.-

tional feedback ,for that purpose, involving a means for rectifying a portion ofthe oscillator.

outputwave anda means for utilizingtheresultant product to conditionthe operation of the circuit element which performs thegain function,

has been reorganized and elaborated forthe purposes of theinvention and therefore with ref.- erence to the tendency' for thedevelopment of such low frequency disturbanceswhile being in More specifically, the conven herentlyineffective to modify ,the'normal genysis which tion in. its various specific forms results ina rathergeneraland simple concept of oscillator performance.

eration ottheprime (carrier) wave, The analinducecl the conception of, the inven- The invention maybe better-understood reference to the accompanymg drawings, in

which: i

of the prior art; here used Fig. 1 discloses a thermally limited oscillator velopment. of. the analysis which eventually led up to the novel: cirouitsdisclosed in later num ,beredfigures; w l

Figs. 2 and 3' are circuit/of Fig.1;8 M l H i Fig; 9 illustrates, comparablyrwith Fig2l1,-; a

secondold circuit asa preliminary to an analysis" thereof. similarly as intherinstance; of;sai.dFig. 1; A

*Fig lllis adiagram disclosing thetransmission characteristics of .a.tuned circuit preliminarily .1 i i to theuse. of thesecharacteristicsin astudyof a thecircuitofflthe next figure 1.1 discloses diagrammatically a circuit of the applicabilitywof, the

analysis covered by :the .earli'ernumbered figures to the stability control of a given oscillator which is itself old in the art, this figure illustrating. the modification specifically claimed J in the parent applicationbut retained hereinias assisting in an understanding of thespecies herein to beclaimed which is illustrated by Figv 14 corresponding; to Eig.-15"of the parent applicationy. i

Fig. 12 comprisesa-diagram teaching thejtrans I mission"characteristics of the auxiliary filter shownin block insaid Fig. 11;:. i i Fig. 13 shows the continuitylof said auxiliary filter contemplated by Fig. 12; and Fig. l iillustratesa circuitycomparable with that of 'Fig. 11, but illustrating the applicability, ofthe basic analysisto the stability: control of athermal limiterttype of oscillator.

Preliminary to a description of the invention l as expressed in its various specific forms, a lponsideration will be given of thebasic' analysis and reasoning leading to an characteristics.

possible the most simple and, effective treatment of thekanalysis,

circuit of one species of thefinvention andthereforea circuit to which the improvement -ot the invention is applied. The analysis isapplicable by a logical extensionflof reasoning, or by anal ogous reasoning, to the other speciesof tha invention and to oscillation generating circuits generallyw m m -.-.Consideration of oscillator circuits in general indicate that onlythreebasic functions arejp er formed in the generation of oscillations; this sugto illustrate the de-f a blockwdiagrams illustrating;

a for purposes of analysis,thezcircuit of Fig.1;

Figs. 4 to 8 comprise analytical curves illus-J trating 1 the performance; characteristics act the i the; 6f

understanding of the inventions' and a prediction aswtoI'its operational For this purpose a typeof,,oscillatorfcircuit will bechosen whichiwill= mal;e

this circuit: being also, the basic tion. In

elements although, as will be apparent later, certain of these functions may be combined in a single element. This is true of all oscillators but thelspecific analysis herein contemplates that a feedback oscillator be used.

First, gain must be provided in order to over- .come circuit losses and to supply a useful load.

electronic the fed back oscillations may be caused to perpetuate themselves. Sometimesthe tuned circuit or filter performs this function, Inpursuance of its frequency determining or stabilizing function, it is necessary that the rate of change of phase of the current passing through it be large for a given change of frequency.- Thirdly, an amplitude-limiting device is necessary. This function is frequently performed by the electron tube used as the gain device, but in some useful forms of oscillation generating circuits it is sepa rate therefromj, and a circuit in which it isseparate, and in fact in which all three elements are separate from each other, lends'itself more easily to the present analysis. This limiter is necessary since all useful oscillators show considerable excess gain and the excess must be accounted for by the limiter in order that a finite constant output shall be achieved. For high amplitude stability the rate of change of loss with be large at the operating respect to level should level. 'The well-known Hartley oscillator illustrates an instance in which the vacuum tube combines the functions of gain and limiter. In the Meacham bridge oscillator of Patent 2,163,403, June 20, 1939, the functions of limiter and filter are both performed by the bridge. 'In Fig. 3 of Caruthers 2,066,333,.lanuary 5, 1937, all three elements are separate, the bridge performing the limiter function only. A practical advantage of the separation of the gain and limiter functions is to make possible the beneficial use of a linear gain element. It is convenient in the present analysis to assume such linear gain element which, because of itslinearity requires no special treatment here so that the discussion will be'co'nfined" to the performance 'of limiters and filter elements Itiswell known that oscillators of all kinds tend to suffer from low-frequency disturbances, that is, the output is modulated in amplitude ata frequency low compared with 'thenormal output frequency. Although sometimes modulation is of relatively small amplitude and nearly sinusoidal, usually it is so violent as to cause complete cessation ofoscillations for a large portion of the time; This phenomenon is likely to have its origin in the limiter. It should be recognized that there are various types of limiterssuch as, (1) thermistors, which change their resistance with the current flowing through them by thermal action, including carbon and tungsten lamps, (2) varistors, which change their impedance with a feedback oscillator comprising an gain element or device, which produces a phase shift, there must be a compensatory phase shift elsewhere in the circuit in order that I of the first and last of The devices of these nomena ofthis kind. The reason for'this is, in

usually, part of the output is promote nearly resembles Fig.

I 2,379,694 gesting the corresponding use of three functional change in the instantaneous current flowing through them, including copper-oxide andthyrlte elements and certain electronic diodes, (3)

vacuum tubes or other electronic devices which reduce their gain by simple overloads or increase of current through them as in the above Hartleyoscillator, and (4) vacuum tubes or the like which reduce their gain with application of a bias developed as a result of the oscillation, wherein,

rectified and re- In general, the devices these four classes are capable of functioning without appreciable harmonic production. in the instance of the thermistors because of the thermal lag, which tends to make them immune to effects occurring within'the cycle time of the frequency concerned. two classes are therefore known as linear elements, and those of the other twoclasses as non-linear elements. Although, as above indicated low frequency disturbancestend to affect oscillators of all sorts, oscillators using thermal limiters, as in the Meacham bridge-0's cillator, that has been mentioned and to which applicant's invention in one species isapplicable, are particularly subject to low frequency pheturned as bias potential.

a generalway as follows; When the circuit is first energized the lamps (thermal limiters) are cold and the loop gain high. Oscillations build up to a large value before the lamps can heat up and a large amount of energy is stored in the oscillatory circuit, In a fraction of a second the lamps heat up but, because of the stored energy, they are carried not to the balance value but to a resistance so high as to cut off the oscillation. If the oscillation dies out before the lamps cool to a'value permitting it to rise again'the cycle will repeat indefinitely.

The basic analysis, as applied in various ways a great variety of oscillators, was developed in connection with a lamp stabilized oscillator, that is an oscillator using a lamp or lamps as the thermal limiter element, because the principle is most easily applied'to it and other typesof linear oscillators. Although applicable to'mo'st other forms of oscillators, the interpretation of the results asapplied thereto may be diflicult in some instances although a knowledge merely of the nature of the problem is often quite useful.

The prototype oscillator for which the method to be outlined below was developed is shown in Fig. 1. This circuit type, as disclosed in the above Meacham Patent 2,163,403, June 20, 1939, except that in order'to ambient temperature although with a'certain sacrifice of corresponding frequency'stability, the filter element is separated from the stabilizer of limiter element. To this extent the circuit'more 3 of C'aruthers;2,066,333, January 5, 1937, also above mentioned. The stabilizer or limiter I is a bridge comprisingresistanceelements' and thermistors, in this in stance, tungsten lamps. This stabilizer is in the feed back circuit of the 2 with tuned circuits 3 and 4 separating the feedback circuit proper from the output and inputcircuits, respectivelyof said amplifierand together constituting what is denominated the filter in the present analysis. In View of the simplicity of the circuit and the limited occasion for ref erence to collateral features of the circuit, fur

' ther description and'use of reference labels is dispensed with.

is of the Meacham bridge amplitude stability with changes "in amplifier or gain device ofiits. input value and. the phase of each isshifted 45 degrees. 3 Thus the envelope falls to two such. circuits. Here for that is,for

in terms ofthe frequency scale of Figs. 4 and 5 Transmission 2 2 2 and l =45 degrees+45 degrees=90 degrees. For

largevalues of the phase approaches 180 degrees.

Since the amplifier and stabilizer taken togetherhave substantially no phase shift, except ,the inherent 180-degree phase shift of the amplifier and since the requisite additional ISO-degree phase shift is achieved by the transformers, or either of them, the normal oscillation must occur substantially at the point of Zero phase shift of the filter. It is significant that this filter can produce an increase in modulation only in the improbable case-of oscillation at a frequency other than; that of maximum transmission. Since it is believed that that case is of no practical interest, decrease of modulation may always be expected in such a device so that the device may be thought of as a fi-path;

. ,The operation of the stabilizer element is much less familiar than is that of the filter. Fig. 6 illustrates the direct current characteristic of a bridge network as shown in Fig. 1, plotted between the impressed potential V and the potential e derived from the alternate vertices of the bridge andfed back to the input of the gain. element. At high frequencies, that is at frequencies above aboutl kilocycle, the characteristic is virtually identical with the direct current characteristic because the thermal-resistance characteristics. are unable to follow the high frequen cy cycle. We wish to evaluate the effect of applying a low frequency modulation of infinitesimal magnitude to such a high frequency Wave. Three possible operating points for the bridge are designated A, B and Cfor convenience in ref erence. The operating point is determined by the voltage of the high frequency wave which in turn is determined by the gain of the amplifier. Evidently one analysis must be made for that voltage which makes'the bridge loss equal to the amplifier gain less the filter loss.

One point of the envelope frequency-transmission characteristic may be evaluatedimrnediameasure of the dependence of the output on the input.v Use is made of this direct current characteristic, in this instance, where it could not be used in the previousinstance because at thelow envelope frequency concerned the thermal lag is such that the thermal-resistance char actristics are able to follow this low frequency cycle. For operating pointB, corresponding to the maximum output, the slope is zero so that the output is independent of input and therefore the envelope transmission is zero. For point C the envelope is transmitted without reversal of phase butsince the slope is less than that of the straight line, the transmission is attended by some loss.

For point A, a more desirable operating point, r

the envelope is reversed in phase since the slope is reversedand thereis a considerable increase in magnitude since a small percentage change of input results in a large percentage change of out: put.

For a fairly high modulating frequency (perhaps .100. cycles) the lamps are able to change their resistance by only a small amount during each modulation cycle. Moreover, the lamps will reach maximum resistance when the modulation is approaching its zero after going through a maximum. Therefore, the attenuation of. the bridge changes only a small amount during the cycle but lags the applied voltage by. nearly degrees. Accordingly, a small component will be added to the outputin a 90 degree leading position.

We now know that the transmission for zero modulating frequency is on the real axis and that itsvalue depends on the operating point on the curve of Fig. 6 such as A, B, or C. We also know that as the modulating frequency approaches a high value the transmission approaches 1,0 at right angles to the real axis.

We know that simple electrical networks which have the transmission points just defined describe a semicircle in traversing the frequency range from low to high. Such a semicircle is shown in Fig. 7 for a particular operating point near A for a particular. bridge. The argument here presented is verified by the fact that a semicircular locus has been observed experimentally for a thermistor bridge similar to this one. InthisFig. 7, the modulation, that is the envelope, frequency points are indicated by the quantities opposite the arrows. The vector values are in terms of the ratio of side-band, transmission to carrier transmission, this being ordinarily thought of as a measure of envelope transmission. It is, important to observe that the percentage modulation of a low frequency test wave .may be greatly increased in the lamp bridge. In fact it is the z ero frequency extrapolation of this behavior which gives use negative feedback and output stabilization in the Meacham bridge oscillator. It is also this increase orgain in'thislenvelope which causes this diagram therefore representing the the diagrams of Figs. 5

. of the given .Figll circuit. Inparticular,

locus loops the (1,0) point, a condition ofin Fig. 8 must then look essentially like trouble. The amplifientan active circuit, has no open and it will be easier to obtain margin in efiectonthe envelope. The filter'n'ormally attenuates the envelope and we have the paradox of a s e ducing all the gain in the system. This anom+ alous behavior is the key to the entire situation. In view of this consideration, the stabilizer has to be treated as a gain portion ofcircuit, hence it is proper to think of the diagram as a y. diagram,

In Fig. 8, the vectorial product s is plotted, vectorial product of the values for o no and '7. That is, for each particular modulation frequency f the vectors of Fig. 5 and Fig. 7 for for that frequency are multiplied to obtain a dissipating c1rcuit, the lamp bridge, pro

point on Fig. 8. This diagram is the end product of the method of analysis and, depending on the precise dimensions of the diagram, which itself depends on the particular electrical values used, it should be possible to predict the stability or instability with reference to extraneous frequencies, if the stability is indicated.

Suppose in Fig. 5

cycles so that e The four indicated points are A A1. 1 rf= 8, 16,32 cycles. Now for f -16 cycles we have, from Fig. 7,

o #:3 80 (that is, with radius vaor 3 at an: angle of so degrees) and, fromFig. 5, 1

e t/ 90 so p At8cycles i H renam -viii. so

. w /g: Clearly t loops the point 1,0 and modulation will result. if e j Alternatively suppose that Q=100 and IF 100,000 sothat I i The frequencies fortheindicated points of Fig. 5

are now i=0, 250, 500, 1000 cycles. For all these frequencies [L from. Fig. '7 isalmost identically unity. Likewise for frequencies'up to 100 cycleslp of Fig. 5 is virtually unity.. ,The resulting plot for ts corresponding to Fig. 8 does not enclose 1,0 but does come very near. to cutting that point.

As a third case let Q=40,000 and F=4,000, a situation that might exist with quartz resonators. The values of f in Fig.5 are now 0, V10; and cycles. For this range of f the a of Fig. "7 is essentially constant at. i v t 1 Fig. 5. For this third case a separates the loopfrom 1,0.. t

In general, if the two resonant circuits have different values of Q a reversal of i very real margin oscillator.

length partly because the choicejof the circuit element, namely, in vacuum The analysis so far has been with reference to a particular circuit in which there is a complete i physical separation df'the elements having the.

various functions pointedout near the beginning of the specification as inherent in anyfeedback The treatment has been at some lends itself well tothe elaboration of theanalytical treatmentand partlybecause theltreatment with i rather obvious variations is applicable quite generally so that the treatment in this place would similar treattendto obviatethe necessity of a ment for other types of oscillators that would be.- come of significance later.

specific application of the invention. Since another specific application of the invention is with reference to adistinctly different typeof basic in brief of such circuit will be introduced of the invention.

Fig. 9 illustrates, rather schematically, avery simple oscillator having amplitude limitation as achieved by a grid bias control. Thisisthe lastmentioned of i the four types of, Iimiterslisted early in this specification. From its similarity to afamiliar radio circuit it is often denominated as the automatic-volume-control (A.-V.-,C.) oscillator. In it the functions of gain and stabili zation or limitation are perforniedin a single tube orthe like 5. Said tube 5,'with the frequency determining-circuit 6 corresponding to the filter in thecircuit above treated, and the feedback from its coil l to the coupling coil 8, provides a conventional tuned output feedback oscillator.

achieved by a second feedback path involving the coil 9, likewise coupledtosaid coil lby which a work comprising theresistancer and capacitance; c and the feedback coil 8. It is evident that the potential across resistance r is afunction of the output amplitude so that it reflects a potential lthereacross in such a way as not only to initially limitftheamplitude which otherwise tends. progressively to get indefinitely.

higher with. each cycle of energy fed back, but

' also tends to control the volume as affected by extraneous.phenomenarefiected in a change of output potential. The capacitance c in general functions to by-pass. all alternating potentials at that point and therefore insure that the potential impressed on the control electrode from. the

resistance r is steady, although the operation considerably depends on the time constant of the re. circuit as affected by the relative values of the The biasing source tial under which the stabilization or gain control does not function. Also by its use a larger change i in the voltage across 1- fora decibel changein the oscillating level is. achieved. Therefore, a larger change in tube or circuit gain maybe tolerated. without unduly affecting the output level. That.

is doubling the number of turns on coili9 at the the 10015013353- 5 willibe 5 am m ubl g t v lta e oi battery II will Also the particular circuit chosen is to be used later to illustrate a i The 1 Stabilization because ofthe necessary amplitude limitation is makes possible a variation of the control electrode potential which three rather "than four quadrants.

not affect the threshold at which voltage begins to appear across r but will double the voltage across r produced by a small fractional rise above this'threshold level.

It would be quite appropriate to include here block diagrams applicable to Fig. 9 and analogous tuned circuit, has already been discussed in connection with the preceding circuit. The new fea-' ture is thebehavior of the amplifier as affected by the gain adjuster elements 10, ll, r and c.

For any reasonable frequency, the gain of the amplifier is a direct function of the bias voltage, without phase or time delay. Accordingly, it is necessary'only to evaluate the behavior of the rectifier ill to a modulated wave and this-is the familiarproblem of the radio detector. Asthe modulation frequency varies from low to highthe corresponding diminution ofenergy of the wave of modulation frequency resulting from detection of themodulated wave varies from zero to infinity and'the phase changes from 0 to 90 degrees. This penom'enon is well known in the art, 'a treatment of it at least to some extent being included in'Bode 2,123,178, July 12, 1938. Applying these' considerations to the transmission of the "modulated wave, 0a,. diagram for the envelope transtwo" fundamental conditions. It must deliver a directcurrent biasthat increases wit-h crease of level of operation and it musta'tte ate to'a high degree any current of oscillation re: quencywhich may enter it. j lhense of al anced or push-pull rectifier contributes 1a ely to the latter without seriously afiectihgthe former. Its use is therefore indicated. "A con stant counterelectromotive force o'r back biasfis conducive toth'e former as has "been explained, and does not-affect the latter. 'It will also-be used. The use of a-counter electromotive forc'e does require, however; t-hatfa generous voltage be delivered to the rectifier either 'b'y "a large winding on the output coil or by a separate a'n'i plifier. Since an amplifier is often-desirable the interest of power output and to make -the frequency independent of the load, a"s in the-case of a buffer amplifier, it will be used here. l 1

Assume, to be specific, that the frequency is to be onemegacycle and that the effective Q of the coil-is 100; The transmission of the modulated'wave in terms of the modulation frequency through a tuned circuitas in the scillator of Fig.9, is shown in Fig. 10, which: is similar- -in effect to Fig. 4 although specific to a different i set of electrical values and somewhat differently mission or the modulated wave through the controlled amplifier may be obtained. Such a diagram is not separately included because it eventuates that the-characteristic is identical with the corresponding diagram for the lamp bridge as disclosed in Fig. 7.

In this case the filter consists of only one tuned circuit. Accordingly, the 'p diagram applicable is a semicircle from 1,0 to 0,0 and approaching 0 at -'90- degrees. Therefore the total n diagram although generally similar to Fig. 8 lies only in v Therefore, the risk'of looping 1,0 'ismarkedly decreased.

One importantdifierence' between the "operational characteristics of the two circuits is in favor of the bias stabilizing system. That is, the time constants of "the chosen only over very narrow limits, while correspondingly, "that for the physical TC circuit of Fig. 9, may be varied at will between rather wide limits; .Also with the bias system no large "high frequency gain need exist, "although effective gain somewhere in the loop is necessary to obtain a. large change in bias with small change of level. In the lamp stabilized oscillator, the bridge loss must be compensated directly by high frequency gain. However, the lamp circuit requires fewer electrical elements than the bias stabilized system.

The development of a simple bias-controlled oscillator will now be explained to clarify the material already given and to illustrate certain ideas not yet presented. The prototype circuit will be that of Fig. 9 to'which certain elements will be added to enable the circuit to avoid the tendency of the circuit to be unstable with respect to the low frequency modulating currents inherently set up in the operation of the oscillater. It is obvious from the above analysis that attention may be confined entirely to the gain lamp 'stabilizer'can be adjuster element and its appurtenances, that organized. It is noted that the phase shift at about 1000 cycles is about 10 degrees, the atten uation, at this point substantially zero. Since this is, for our purpose, a very slow cut-off, we may design our gain adjuster to cut off more rapidlyand, provided we achieve a proper coordination "of the 'two,'we, "may realize an adequate margin. i

The circuit features already arrived at are shown in Fig. 11.; The circuit comprises the oscillator l2'with'a single tuned circuit I3, a buffer amplifier l4, havin littleselectivity and th'er'e' fore contributing only a little to the equivalent filter section, a balanced rectifier l 5 with a source 16 of biasing electromotive force and an auxiliary low-pass filter section 11. The analogy to the prototype circuit of Fig. 9 is obvious. Since it will be found, when the auxiliary filter is' later 'described, that it contains direct connections between the upper and lower pairs of terminals, respectively, the capacitance I8 and resistance I9 somewhat closely simulate in function the capacitance cand resistance r of Fig. 9, to which are super-added the circuits within the b10015 and which will be discussed presently. :Said, capacitance I 8 is only large enough to allow the rectifier to be driven without too much loss from th oscillation frequency. It produces negligible effect at frequencies below 50 kilocycles.

Let it be supposed that the b fie mplifier; rectifier, etc., have been so chosen ,that aa: zero: frequencymodulation of one parttoa millinn ane plied at the pl'ateterminal wvillresult'in one thousandparts per: million-returned to that .point. This is equivalent toafiOudeCibels; feedback. :It 1318 proposed that the auxiliary low-pass; filter be given approximately the characteristic shown in Fig. 12.- Ihiscurvei is plotted betweenifrequency On a logarithmic scale and;;attenuation ldfifii bels'. No attenuation atfrequenciesbelow Bpycles per second is called for so that thecircu-itmay not be sluggish. Between 3 and 3000 cycleslper second the loss rises linearly to 60 decibels, a voltcapacitance cult ofv the resistance; Fig-12 shows that a phase curves is nowhereeaslarge as .180

characteristics ihdicated s s .WhiChdiScbses thecircuit included in the auxils l thereactance of cond ohms h and the attenuaegevactil r enough that the impedanceon account of the is negligible in proportion to the cirabout 60 ldecibelsri increases; tunedcircuitlfilter) still is producing very little effect as shownin Fig; 10. iwI-lowever; the :auxil iary filteris lproducing about50*decibels loss and angle ofapproximately .90 degrees is-a'ssociatedfl with such an "attenuation. In the frequencyregionfrom 3 to 300 kilocycles the loss is held flat at 60 decibels; This is desirable in that it, inthe;

particular network used, andwillustratedvby Fig. 13, causes the phase angle to drop back. toward zero. Moreover the modulation picked up in the;

exactly annulled byzthe'modulation resulting from this amount of transmission.

feedback coilis Over this band therefore o is closely zero and no trouble may result. Actually the tuned ,circuit brings in its loss and phase in this interval (Fig;

the sum of the two phase s ,since however,

degrees (which, of course, it would have to be to givean overall 0 phase shift) until the attenuation is veryhigh,

there is considerable margin against looping the,

point 1,0.. In practice any loss between 55and 75 decibels for this flatportion of the loss characteristic] is quite safe since the isnot critical. s A

Elements which will :give approximately the are shown in Fig. 13

iary filter I! of Fig.,1l,and in which the resist ance of resistor 2! is assumed for the practical case usedto be 100 ohms and the capacitance of condenser 2,2. is assumed to be balance pointed out and thephaseshift is only IOOdegrees; s i of envelope v aboveab0ut1'3,000 'cycleslgainycrossing) and that *the phase is no- 10011degrees until frequencies:

The shunt circuit x 2ll is tunedto the one. niega cycle frequency, assumed. Otherwise," it i noncritical. lnthe practical casefthe resistance of resistor l9 was200,000 ohms and that ofresistor The operation of the filtefcircuitis outlined asfollows: At aerofrequjency o); c.) thefshunt arms are ineffective i is delivered to resistor [9. At 3 cycles fper second W tioni's increased bythe shunting eilect of 22 on l9. second conden-ser ZZ offersonly 3,000 ohms'and accordingly the attenuation as compared the3 cycles condition is about r about 30 decibels. this element 22 offers 100 ohms reactance, equal to the resistanee of element 2| and the attendation is about or decibels. At higher frequencies resistor, 2|, controls and the loss due to this arm does not change. At a frequency of one megacycle the operating frequency arm 20 is resonant and offers a verylow impedance. Accordingly, the loss at this frequency is very high; Moreover, the impedanceseen byresistor i9 is low so that the cur rents induced in the feedback coil grid of the tube. It will be evident that the network of Fig. 13, teachings ofFig. 12.; l

The over-all envelope as will now be considered further on the basis that the loop has been U opened at the plate terminal of the tube. A low frequencymodulati-on is returned unaifected to At 3,000 cycles per second.

coupled to the coil of tuned circu1tl3 are fully delivered totheand its operation carry out the and 2/3 ofthe applied voltage l 40 enser 22 is equal to 100,000 7 At 100 cycles De output current to beabstracted from the oscillator and rectified in balanced or resistance is steady,

the grid by theffeedback coil. However,the buifer amplifier,*rectifier and, auxiliary filter return a bias such that impedance, show a reversal of modulation and the plate current, into a suitable Clearlyt s q the network of Fig 13 isa direct outcome: of the e aThe slow 'and idegree" phase shift (Fig, L12) Therefore, rthe envelope is r transmitted with Jonly oa decibels ,Atuaooo gainxand about 80 degree phase shift.

cycles the tuned circuit produces about=60. degree phase shift and 1[ It may beseenthatthe magnitude m8 is below 1 for all frequencies where more than above 100,000 are reached. 1 in terms of 'feedback the phase margin is 80 degrees (180 degreesnto degreesy and thegain margin, asl deflned by s i wat leastc20 These are regarded l as generous: inare Fig; -10 for about 100,000 cycles decibelsp e characteristic of Fig. 12 given by system of analysis here taught. controlledycut-off .is in marked'rdistinction to general practice which calls for steep cut-off starting atavery low frequency, l

When oscillators involving: thermal limiters,

in the instance ofthe Figul circuit; are in troue ,ble from low frequency interruptions or the like; i

that is when-they are affected (by the phenomena treated in this specification, it is, sometimes possible to remedy the condition, by application of bias controls Obviouslyl theidirect currentbias,

that is "the not be able to function properly, that frequency. However, low frequencies may be returned through a blocking condenserw in a such a way-as to alteruthelmodulationpcchar acteristic and :thereby promote stability; This is analogousuto adding,"in the p path of an amplifier, elements which produce negligibleefiects in the band but promote phase-margin in the regionot the gain crossin Figrli acircuit. l

1 Except for the novelbias controli ieature,the circuit of Fig. 14 isastandard Meacham oscillater as disclosed in; his Patent ,2,l63,4 .03, June 20, 1939. Aphysical resonant circuitis indicated,

, which, however, maywbe replaced bygazcrystal; e ifl'desired; The circuit tOsWhiCh the-biasacontrol s is applied. is. quite simij larto Fig, 1 also, the differences being-obvious; The bias control feature has some to the nearest corresponding feature (of the Fig.

feature of the invention 11 circuit. For sim licity, in view of the prior disclosure and the disclosure of theMeacham patent, the description here will be confined to this bias control feature.

Transformer 35 enables a desired amount of tance 38 insures that the rectifier would give comparable results. This steady state condition, however, is predicated on a steady state oscillation, that is the generation of a wave withoutmodulation. For this condie tion there ,can be no current flow through the 1 shunting circuit comprising resistance 39 and caiAt 1000 cyclesithe i 6 decibels lossaaridthe auxiliary filter "60' decibels: loss 'andi 40, degree Ephase shift. 1 Thus, the envelope attenuated by decibels f bias with respect to the high he: quency must notbechanged or the limiter would to stabilize at s discloses such resemblance double wave rectifier .36, the resultant potential being im-j. pressed across the resistance 31. The capacipotential across said the double-wave rectifier promoting this result although a single wave trode, a a feedback plinfg the output pacitance 40 so that no potential exists across resistance .39. Accordingly, a potential equal, to that across resistance 31 exists across capacitance. If,..however, the ainplitudeof oscillation is chaneingzas becauseof the incidence of a modulation condition, thencapacitance 40 must charge and discharge Jthrough resistance 39. This increases or decreases the bias and there-. fore correspondingly, aifects the gain of the tube. This effect is enhancedjif the tube is of the var-. iable type, which can easily be realized in practice. The connections, as b the poling of the rectifier relatively to the network to which it is connected, should be such that an increasing level decreases the gain and vice versa. This all means that by the expedient of the invention,

the carrier, that is the desired wave,, is generated in a conventional manner without being af-' fectedbythe superposed gain control circuit but that, because ofthe gain control circuit there is avolume control afiecting the envelope only'and insuring that no envelope is generated.

,The fact of stability of the circuit can'be established by opening the oscillating loop, atthetermin'als'of the input transformer, and inserting a generatoryand a detector as for a s measurement, and according, to the early analysis in this specification. A wave of the desired frequency and level is inserted and, when properly chosen, returns unchanged in amplitude and phase.- A small low frequency modulation frequency-is now added. If the invention is to be eflective, the low frequency envelope must not be! returned in phase with thatsupplied unless at a loss of amplitude. Without the added elements of the gain control, it would be found that this criterioncannot be satisfied. With theloss produced bythe added elements stability is assured. 9

While the invention has been described in connection with particular embodiments, certain variations have been suggested and ittis to be understood that .many additional changesare possible within the scope of theinvention'.

:What is, claimed is: I I "1. A stable frequency,-pure wave,' oscillator comprising an amplifier device having input and output electrodes including a common. elecpath including a frequencycoupling the input and outdevice, and an auxiliary feeddetermining means put circuits or said exclusive of the common said common electrode.

say

back path, at leastin part independent of said lirs't mentioned feedback path and likewise couand hiput eircuits of the .de-.

means for deriving a portion said fconditioning means including a rectifier.

and an amplitude and phase regulating network,

said network comprising in parallel relationa resistance and a serial combination of resistance and blocking capacitance, saidv last-mentioned resistance being connected inthe input circuitioii said amplifier device and said rectifier being-so poled relatively to said network that an;,increase of rectified current tends to result infannincreased negative bias on the input 'electrode electrode relatively to 2. A stable frequency, pure wave, oscillator comprising an amplifier device-having a pair of input and a pair of output electrodes, said pairs including a common electrode, a feedback path between said pairs of electrodes bridge-like network conjugately connected to said'pairs of input and output electrodes said bridgeelike network including a frequency determining means and a current responsiva variable resistance, amplitude control element, and'an auxiliary feedback path, at leastin part independent of said first-mentioned feedback path and likewise coupling the output and input circuits of the device, said auxiliary path comprising a rectifier, and an amplitude andphase regulating network including in parallel relation a resistance and a serialcombination of resistance and blocking capacitance, said lastmentioned resistance being connected in the in put circuit of said device and said rectifier being so-poled relatively to the network that an increase of rectified current tends to result in an'in creased negative bias on said input electrodes relatively to said common electrode becauseof the derived potential across resistance.

3. The organization recited in claim 1 in which said network comprises additionally a'steadying capacitance shunting said parallel resistance.

4. The organization recited in claim 1 in which said rectifier is a double Wave rectifier.

5. The organization recited in. claim 2 in whichsaid rectifier is a double-wave rectifier and in which said first-mentioned network resistance is shunted by a steadying capacitance.

WILLIAM A. E som and includin a a said last-mentioned 

